The present invention relates to the growth of crystals and, more particularly, to the growth of silicon carbide crystals.
Over the last decade, the use of silicon carbide as a semiconductor material has grown dramatically. Silicon carbide semiconductors have certain properties, including a wide bandgap, high thermal coefficient and capacity to operate at far higher temperatures than certain common semiconductor materials, such as silicon, which are desirable in various semiconductor applications.
In many applications, it is desirable to use an underlying semiconductor material that is substantially of a single polytype (silicon carbide can, at least theoretically, form at least 150 different types of crystal lattices or xe2x80x9cpolytypesxe2x80x9d). Additionally, it is typically preferable that the semiconductor material have low levels of defects in the crystal lattice and/or low levels of unwanted impurities. In fact, even in a pure material, a defective lattice structure can prevent the material from being useful for electrical devices, and the impurities in any such crystal are preferably controlled to give certain desired electrical characteristics (such as an n or p character). As such, the availability of appropriate silicon carbide crystals is generally desired for the successful manufacture of electrical devices from silicon carbide. Additionally, because of cost considerations, it is desirable to grow relatively large silicon carbide crystals, from which a large number of xe2x80x9cwafersxe2x80x9d may be produced. Cost and device specific considerations also make it desirable that these wafers have a relatively large surface area.
Traditionally, two broad categories of techniques have been used for forming crystalline silicon carbide for semiconductor applications. The first of these techniques is known as chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) in which reactant gases are introduced into a system to form silicon carbide crystals upon an appropriate substrate. The second main technique is generally referred to as sublimation. In this technique, some type of solid silicon carbide material is generally used as a starting material. This starting material may be of one or more different polytypes, and may or may not include particles of the same polytype as the polytype which is desired for the single crystal semiconductor material. The solid silicon carbide starting material is heated in a crucible until it sublimes, and the vaporized material is encouraged to condense, with the condensation intended to produce the desired crystal. Typically, this is accomplished by introducing a monocrystalline silicon carbide seed into the crucible and heating it to a temperature less than the temperature at which silicon carbide sublimes. A pioneering patent that describes methods for forming crystalline silicon carbide for semiconductor applications using such sublimation techniques is U.S. Pat. No. 4,866,005 to Davis et al., issued Sep. 12, 1989, which was reissued as U.S. Pat. No. Re. 34,861, issued Feb. 14, 1995, which patents are incorporated herein by reference as if set forth in their entirety.
According to embodiments of the present invention, methods of growing silicon carbide are provided. Pursuant to embodiments of the present invention, these methods use an electric arc to sublime a silicon carbide source material. In these embodiments, a silicon carbide seed crystal is introduced into a sublimation system, along with first and second electrodes that are separated by a gap. A power supply is coupled to at least one of the electrodes and used to create an electric arc across the gap between the two electrodes. This electric arc is used to sublime at least a portion of a silicon carbide source material. The vaporized silicon carbide material may then be encouraged to condense onto a seed material to produce silicon carbide. In embodiments of the present invention, at least one of the electrodes is comprised of silicon carbide and serves as the silicon carbide source material.
In specific embodiments of the present invention, methods of growing silicon carbide are provided in which a silicon carbide source is electrically arced to sublime silicon and carbon containing material from the silicon carbide source and cause at least some of the silicon and carbon containing material to form silicon carbide on a silicon carbide seed. In certain of these embodiments, the electrical arc may be established between a pair of spaced apart silicon carbide electrodes. In forming silicon carbide pursuant to these methods, the power dissipated across a gap between the pair of spaced apart silicon carbide electrodes may be controlled to control the flow of vaporized Si, Si2C and SiC2 from the pair of silicon carbide electrodes to the silicon carbide seed. In certain embodiments, this flow of vaporized Si, Si2C and SiC2 per unit area per unit time from the pair of silicon carbide electrodes to the silicon carbide seed is controlled to be substantially constant.
In other embodiments of the present invention, the power dissipated across the gap is controlled by moving at least one of the pair of silicon carbide electrodes as they vaporize during the sublimation process to maintain a constant gap between the pair of silicon carbide electrodes. In specific embodiments, this control of the power dissipated across the gap may be accomplished by sensing the voltage drop across and/or the current through the gap and adjusting the relative location of the silicon carbide electrodes so as to maintain the voltage drop at a constant level. In yet other embodiments, the pressure within the sublimation system may be maintained at a substantially constant level during the sublimation process. These sublimation processes may occur within a heated furnace, and the internal temperature of the furnace, the position of the pair of silicon carbide electrodes, the voltage drop across the spacing between the pair of silicon carbide electrodes and the arc current may be configured so as to maintain the ends of the pair of silicon carbide electrodes adjacent the arc at a substantially constant temperature during the sublimation process.
In still further embodiments of the present invention, methods of growing silicon carbide are provided in which a furnace is heated to a temperature below the temperature at which silicon carbide sublimes, and a local high temperature zone is created within the furnace that is above the temperature at which silicon carbide sublimes. In these embodiments, a silicon carbide source material may be introduced into the high temperature zone to sublime silicon and carbon containing material from the silicon carbide source and cause at least some of the silicon and carbon containing material to form silicon carbide on a silicon carbide seed.
In other embodiments of the present invention, methods of growing silicon carbide are provided in which a seed of silicon carbide, a silicon carbide electrode and a second electrode are introduced into a sublimation system. The electrodes are positioned such that they are separated by a gap. In these embodiments, an electric arc may be established across the gap between the silicon carbide electrode and the second electrode to vaporize at least part of the silicon carbide electrode and cause at least some of the vaporized silicon carbide materials to form silicon carbide on the silicon carbide seed.
Sublimation systems which may be used in performing these methods are also disclosed herein.